The present invention concerns the improvement of the properties of polyvinylidene fluoride (PVDF) and, more particularly, of its flexibility, on contact with hydrocarbons. This improvement is contributed by incorporating an elastomer into the PVDF. By "PVDF" is meant not only the homopolymer, but also the copolymers obtained by copolymerization of at least 70% by weight of vinylidene fluoride with another fluorinated monomer. However, the homopolymer is preferred.
PVDF is a semicrystalline polymer whose essential mechanical, electrical, thermal, permeability and chemical resistance properties depend on its crystallinity. However, after the PVDF is placed in service this crystalline state is the cause of the low elongation at breakpoint and of stretching at the flow threshold. These drawbacks prevent the use of PVDF in a certain number of applications and, in particular, in the use of PVDF pipes for use with hydrocarbon streams.
In order to remedy the drawbacks cited, it has been proposed to graft a nonfluorinated ethylene compound to the PVDF homopolymer or to copolymerize it with the vinylidene fluoride. Although the drawbacks noted are diminished, they are unfortunately compensated for by a loss of other soughtafter properties of the PVDF.
In the special application of PVDF to the transport of hydrocarbons, such as, for instance, the pipes serving for the extraction of petroleum products, it has been proposed as in French application (FR.A) 2,560,884 to incorporate liquid plasticizers into the PVDF; a plasticizer, for instance, of the polyester type. This technique makes it possible to improve the elongation at the flow threshold of the PVDF and consequently to fabricate petroleum hose without the risk of breaking and processing a sufficient flexibility. With this pipe operating several years the hydrocarbons extract and carry away the plasticizer, an then one observes that the PVDF has lost its flexibility, resulting in the breaking of these pipes under high bending or stress.